Colorectal cancer is the second commonest cause of cancer death. Some people have an inherited defect in the genes which repair DNA which results in a very high risk of colorectal (bowel) cancer at a young age. This syndrome is called hereditary non-polyposis colorectal cancer (HNPCC) or Lynch syndrome. Colonoscopic surveillance of HNPCC patients has been shown to reduce the risk of colorectal cancer and allow detection at an earlier stage, but even with meticulous examination, some precancerous lesions or cancers are missed.